Method for the surface treatment of aluminum electrodes for the electrolytic production of zinc,and electrodes thus treated

ABSTRACT

A METHOD IS DISCLOSED FOR THE SURFACE TREATMENT OF ALUMINUM ELECTRODES FOR THE ELECTROLYTIC PRODUCTION OF ZINC, CHARACTERIZED IN THAT IT SUBSTANTIALLY CONSISTS IN DIPPING SAID ELECTRODES, BEFORE THEIR USE, INTO A BATH COMPRISING AN AQUEOUS SOLUTION OF AT LEAST ONE SOLUBLE LEAD SALT. THE PREFERRED SOLUBLE LEAD SALTS ARE LEAD FLUOSILICATE AND LEAD FLUOBORATE.

UnitedStates Patent 3,689,385 METHOD FOR THE SURFACE TREATMENT OF ALUMINUM ELECTRODES FOR THE ELEC- TROLYTIC PRODUCTION OF ZINC, AND ELEC- TRODES THUS TREATED Giovanni Scacciati and Giovanni Lanfranco, Turin, Italy, assignors to Centro Ricerche Metallurgiche S.p.A., Turin, Italy No Drawing. Filed Nov. 30, 1970, Ser. No. 93,893 Claims priority, application Italy, Dec. 2, 1969,

' 25,228/69 Int. Cl. B01r 1/00; C22d 1/22; C23c N08 US. Cl. 204-114 9 Claims ABSTRACT OF THE DISCLOSURE A method is disclosed for the surface treatment of aluminum electrodes for the electrolytic production of zinc, characterized in that it substantially consists in dipping said electrodes, before their use, into a bath comprising an aqueous solution of at least one soluble lead salt. The preferred soluble lead salts are lead fluosilicate and lead fluoborate.

The present invention relates to a method for the surface treatment of aluminum electrodes used for the production of zinc by electrolysis, in order to eliminate or at least reduce to a minimum the detrimental effect of fluorine present in the electrolytic solution. The invention also relates to the aluminum electrodes thus treated.

It is quite well known that one of the most difficult problems in the metallurgy of zinc by electrolysis arises from the presence of fluorine in the electrolyte. In fact, when the fluorine content exceeds certain values, there occurs the phenomenon of the adhesion of the zinc deposits to the aluminum cathodes; the adhesion may be such that it becomes exceedingly difficult or even impossible to bring about the detachment of the zinc deposite; or, as it is commonly called, the stripping of the zinc deposit.

Since the greatest part of the zinc ores contains fluorine in more or less great quantities, the problem is common and always present in all the electrolytic zinc producing plants. There often occur production losses and, at any rate, the control of the fluorine invariably represents a costly and different problem. Moreover, certain zinc ores, quite excellent from all other points of view but containing high percentages of fluorine, cannot be accepted for the metallurgical treatment via electrolysis.

The phenomenon of adhesion is attributed to the pickling action that the fluorine present in the electrolyte ex-' erts on the surface of the aluminum in the starting period of the electrolysis; that is, when said surface is not yet completely covered by the depositing zinc. The pickling action consists in destroying the oxide film that exists on the surface of the aluminum and that hinders the adhesion of the zinc deposit.

This phenomenon occurs in a quite different manner in the various plants, depending on the electrolysis conditions, and is probably related to numerous other factors.

At any rate, so far it has been impossible to correlate the action of the fluorine with its concentration in the eelctrolyte; for example, there exist plants which cannot tolerate more than -20 mg./l., while others may operate with 200 mg./l. and more of fluorine.

At any rate, a sudden increase of fluorine, resulting in contents above the customary levels; causes the phenomenon of the difficult or impossible zinc stripping operation.

Different proposals for elminating the detrimental effect resulting from the fluorine have been studied and applied, but none so far has yielded results satisfactory under all "ice circumstances. One of the most frequently used methods is that of dipping for some time the electrically insulated aluminum cathodes into the electrolytic cell; however, this expedient is often inadequate and any way quite burdensome because it disturbs the normal operation of the electrolysis and results in higher labor costs.

Thus, an object of this invention is to provide a method for the surface treatment of aluminum cathodes such as to eliminate in all cases, or at least to reduce to a minimum, the harmful effects of fluorine, as described above.

Another object of this invention is to provide a cheap method, capable of easy fulfillment, for the treatment of aluminum cathodes.

These, and still other objects which will become still clearer from the following description, are conveniently achieved by a method for the surface treatment of the aluminum electrodes for the electrolytic production of zinc; said method, according to this invention, substantially consisting in dipping said electrodes, before their use, into a bath including an aqueous solution of at least one soluble lead salt.

Amongst the various soluble lead salts that may be used may be mentioned merely by way of example lead fluosilicate and lead fluoborate. Preferably, the lead salt solution is acid and this acidity is imparted by the presence therein of at least one acid corresponding to the lead salt used, and preferably selected from the class consisting of fluosilicic and fluoboric acid.

The dipping times may vary Within a wide range, depending on the desired effect, for instance, from 5 to 500 seconds. For reasons of maximum productivity, preference will be given to dipping times as short as possible.

Likewise, the concentration of lead (Pb ions) in the treating bath may vary within rather wide limits; thus, for instance, use can be made of a bath having a lead concentration between 0.05 and 5 g./l., in the presence or absence of free fluosilicic acid or free fluoboric acid in a concentration between 1 and 10 g./l.

The protective action which the above-indicated treating bath develops on the aluminum electrodes, thus avoiding or reducing to a minimum the etching by the fluorine contained in the electrolytic solution and the consequently high adherence of the zinc deposited on said electrodes, probably takes place in the following way:

The dipping of the aluminum into the treatment bath containing Pb++ ions causes the formation on the surface of the aluminum of a thin, discontinuous layer of lead cement according to the reaction:

At any rate, the layer of lead cement thus produced is only slightly adherent and does not cover the aluminum surface in a uniform and continuous way, but merely spotwise. The lead cement layer sets up a kind of cushion between the aluminum and the zinc forming thereon during the electrolysis of the zinc solution, thereby hindering the direct contact between the two metals (aluminum and zinc) and thereby facilitating their subsequent separation. The protective action of this lead cement layer is extremely effective Whatever the content of fluorine in the electrolyte may be.

It is preferable to avoid establishing an excessively thick layer of lead, for this may cause premature detachment of the deposited zinc. The quantity of lead that, on completion of the treatment according to the present invention, remains adhering to the aluminum cathode varies, of course, depending on the concentration of Pb' in the bath and on the dipping time.

For purposes of exemplification it should be noted that effective deposits proved to be those containing the remarkably small quantities of from 0.02 to 0.2 g. of lead per square meter of treated electrode surface.

When the stripping opera-tion is carried out, part of the lead that has been deposited on the aluminum electrodes is removed by and with the zinc plate and so pollutes the zinc metal thus produced, adding itself to the lead already present therein. However, the quantities of lead thus involved will only constitute about 5 to 20 p.p.m., and therefore acceptable in most instances.

It should also be borne in mind that it is not necessary to repeat the dipping into the lead-containing bath for each electrolytic cycle; the effect of one treatment may be enough for 2-3 subsequent cycles. The extent of the lead pollution of the zinc is thus reduced.

From the electrochemical standpoint the layer of lead cement on the electrodes is not detrimental to the depositing of the zinc, the overvoltage of hydrogen on lead being about the same as that on zinc or aluminum.

The corrosion of the aluminum cathode caused by the method of treatment according to this invention is quite irrelevant. In fact it has been found that for an aluminum electrode having 1 sq. m. of total immersed surface and for a treatment every 48 hours, in one year a loss of about g. of aluminum occurs based on a total weight of the immersed part of 5-6 kg. Thus, the treatment does not significantly affect the normal average life of the aluminum electrodes.

In industrial practice, after each electrolytic cycle (generally during either 24 to 48 hours) the set of electrodes is carried to the manually or automatically operated stripping station, Where the zinc deposits are stripped from the aluminum cathodes; subsequently the cathodes are washed with water and again arranged in the electrolytic cells.

The dipping into the treating bath according to this invention is made after the stripping and before the waterwashing. Since the dipping treatment only takes a few seconds or at most a few ten seconds, it affects the electrolytic cycle to a quite negligible extent.

In order still better to illustrate the present invention, a few examples are now given. These have been taken from a series of tests performed by feeding 4 industrial electrolytic cells, arranged in cascade, with the electrolyte for zinc additioned with an established quantity of fluorides. Each cell had 6 aluminum cathodes. The average operational conditions were:

Feed into first cell-Zn=80 g./l.; H SO =90 g./l. Outflow from last cell-Zn=60 g./l.; H 80 120 g./l. Temperature-40 C.

Current density-5000 amp/m).

EXAMPLE 1 The feeding solution contained 50 ing/l. of fluorine. Three cathodes of each cell were treated according to the present invention by dipping them for 120 seconds into a solution containing: lead-0.5 g./l.; free fluosilicic acid-1.8 g./l.

The cathodes were then washed with a jet of water and subsequently employed in the electrolysis.

The dipping treatment was repeated after every three strippings.

The stripping operation of the zinc deposited on the pre-treated cathodes was easily done.

The stripping of the deposits from the non-pre-treated cathodes of the same cells was very difiicult if not impossible.

EXAMPLE 2 The feeding solution contained 50 mg./l. of fluorine. Three cathodes of each electrolysis cell were treated according to this invention by keeping them immersed for seconds in a solution containing: lead-0.1 g./l.; free iflllOSiliClO acid-1.3 g./1.

The cathodes were then washed with a jet of water and subsequently employed in the electrolysis. This treatment was carried out on alternate days.

4 The stripping of the zinc deposited on the pre-treated cathodes was easy. The stripping from the non-pre-treated cathodes of the same cells was very difficult if not impossible.

EXAMPLE 3 The feeding solution contained l5 mg./l. of fluorine. Three cathodes of each electrolysis cell, after having been washed with water, were treated according to this invention by dipping them for seconds into a solution containing: lead-0.5 g./l.; free fluosilicic acid-l5 g./l.

After the treatment, the cathodes were washed with a jet of Water and were then employed in the electrolysis.

The stripping of the deposited zinc from the pre-treated cathodes was easily carried out. The stripping from the non-pre-treated cathodes of the same cells was definitely more diflicult.

Results of a comparable nature are obtained when free fluoboric acid is substituted for the free fluosilicic acid in the dipping solutions.

What is claimed is:

1. A method for the surface treatment of aluminum electrodes for the electrolytic production of zinc wherein said electrodes, before their use, are dipped into a bath comprising an aqeuous solution of at least one soluble lead salt, and thereafter using said electrodes as a cathode in a cell during the electrolytic production of zinc.

2. A method according to claim 1, wherein the soluble salt is selected from the class consisting of lead fiuosilicate and lead tfluoborate.

3. A method according toclaim 1, wherein the lead concentration (Pb ions) in the treating bath is between about 0.05 and 5 g./l.

4. A method according to claim 3, wherein said solution is acid.

5. A method according to claim 4, wherein the acidity is imparted by the presence in the solution of lead salt of an excess of at least one acid corresponding to the lead salt used.

6. A method according to claim 5, wherein the acid is selected from the class consisting of fluosilicic and rfluoboric acid.

7. A method according to claim 4, wherein the acid concentration in the treating bath is between about 1 and 10 g./l.

8. A method according to claim 1, wherein the dipping time in the treating bath is between about 5 and 500 seconds.

9. An aluminum electrode for the electrolytic production of zinc having a discontinuous, spot-wise surface layer of lead, wherein said lead layer ranges between about 0.02 to 0.2 g. of lead per square meter of the treated electrode surface.

References Cited UNITED STATES PATENTS 1,589,564 6/1926 Robinson 204-281 2,842,461 7/1958 Wagner et a1 117-130 R 1,539,993 6/ 1925 Conley et al. 204-281 2,297,241 9/ 1942 Perner 117-130 R OTHER REFERENCES Kirk, R., and Othemer: Encyclopedia of Chemical Technology, vol. 15, pp. 258-9, New York, 1956, The Interscience Encyclopedia, Inc.

JOHN H. MACK, Primary Examiner R. I FAY, Assistant Examiner U.S. Cl. X.R. 

